The first is almost always the second, but not the other way around.  Just think about it: Almost any time someone gives you an excuse, it is also a reason for what they have done.  They can, however, come up with a wide variety of reasons which will not be acceptable as excuses.  These two are not universally interchangeable.  In our dealings with others, whether it be children, employees, employers, politicians, friends, family, or even God, we must not let these two concepts be confused.